6838
J. Phys. Chem. B 2001, 105, 6838-6845
Characterization of Ultrasmall CdS Nanoparticles Prepared by the Size-Selective Photoetching Technique Tsukasa Torimoto,*,† Hironori Kontani, Yoshihiro Shibutani, Susumu Kuwabata, Takao Sakata,‡ Hirotaro Mori,‡ and Hiroshi Yoneyama* Department of Materials Chemistry, Graduate School of Engineering, and Research Center for Ultrahigh Voltage Electron Microscopy, Osaka UniVersity, Yamada-oka 2-1, Suita Osaka 565-0871, Japan ReceiVed: March 12, 2001; In Final Form: April 27, 2001
Ultrasmall CdS nanoparticles were prepared by the size-selective photoetching technique. The smallest size of the CdS nanoparticles was obtained by the irradiation with the monochromatic light having the wavelength of 365 nm, while the absorbance of CdS nanoparticles monotonically decreased, for the whole wavelength, to null if the irradiation was performed using a wavelength less than 365 nm. The surface modification of CdS nanoparticles with thiophenol caused the red-shift of the absorption spectra due to the electric interaction between the bound thiophenol molecules and the CdS core, the degree being enhanced with a decrease in the particle size. 1H NMR signals of thiophenol bound on the surface of the smallest CdS nanoparticles prepared by 365-nm irradiation were shifted upfield compared to those obtained for the larger thiophenol-modified CdS nanoparticles prepared by the longer wavelength irradiation, and the signals appeared in positions similar to those of chemically synthesized CdS cluster molecules. The observation with transmission electron microscopy revealed that the smallest thiophenol-modified CdS nanoparticles had an edged pyramidal shape with a size of about 1.7 nm in contrast to the larger CdS nanoparticles of almost spherical shape. The particle size and chemical compositions of the smallest thiophenol-modified CdS nanoparticles roughly agreed with those of Cd32S14(SC6H5)36‚4DMF, which has been reported to be one of the largest chemically synthesized CdS cluster molecules. The similarity between the emission and excitation properties indicated that the smallest thiophenol-modified CdS nanoparticles had the energy structure almost equal to that of Cd32S14(SC6H5)36‚ 4DMF.
Introduction Size-quantized semiconductor nanoparticles, which are in the transition region between bulk materials and molecules, have been very interesting research subjects.1-12 Extensive research has been done both in the synthesis of colloidal semiconductor nanoparticles and in the clarification of their size-dependent physicochemical properties.13-39 One of the most exciting questions is how molecular properties remarkably develop when the size of semiconductor nanoparticles decreases. So far, several attempts have been made to prepare the ultrasmall II-VI semiconductor nanoparticles and to control their size and size distribution.23-26,34,38-40 For example, semiconductor nanoparticles of CdS,23,40 CdSe,26,34 and CdTe,24,25 with a size of less than 3 nm were successfully synthesized by thermal reactions of cadmium ions and corresponding chalcogenide compounds in the presence of surface capping agents, and their sizes were controlled by changing the reaction conditions, such as the kinds of surface capping agents and the heating time. In contrast, several kinds of well-defined metal-chalcogenide cluster molecules were considered to serve as a molecular model for solid semiconductors. The chemical syntheses of large cluster molecules have been performed for the purpose of overlapping the properties between the cluster molecules and the semiconductor nanoparticles,41-54 and in the case of CdS cluster * Corresponding author. E-mail:
[email protected]. † Present address: Catalysis Research Center, Hokkaido University, Sapporo 060-0811, Japan. ‡ Research Center for Ultrahigh Voltage Electron Microscopy.
molecules, [Cd32S14(SC6H5)36]‚4DMF48 and [Cd32S14(SCH2CH(OH)CH3)36]‚4H2O50 have been reported to date as the largest structurally well-characterized clusters. We have recently reported the usefulness of size-selective photoetching as a means of preparation of the monodisperse CdS and ZnS nanoparticles of diameter larger than 2.2 and 2.6 nm, respectively.55-61 The principle of the size-selective photoetching relies on both facts that metal chalcogenide semiconductor particles are photocorroded in aqueous solution under irradiation and that the energy gap of size-quantized semiconductor nanoparticles increases with a decrease in the particle size. If the irradiation is performed with use of the monochromatic light, which can induce photoexcitation of the large particles alone, these particles are selectively photoetched to smaller ones until the irradiated photons are not absorbed anymore in the nanoparticles, due to the size quantization effect. This technique could also be applied to the preparation of semiconductor nanoparticles in the cluster molecule regime, in principle, because the size of the semiconductor nanoparticles are simply controlled by the wavelength of the irradiation light. To our best knowledge, however, such attempt has never been performed. Furthermore, the size-selective photoetching technique has an advantage for the systematical investigation of the size-dependent physicochemical properties, because a fixed preparation method of semiconductor nanoparticles seems to be suitable to prevent the unexpected changes in the surface conditions caused by the use of the different kinds of the stabilizing agents and/or reaction species.
10.1021/jp0109271 CCC: $20.00 © 2001 American Chemical Society Published on Web 06/20/2001
Characterization of Ultrasmall CdS Nanoparticles In this study, we have investigated the wavelength range available for the preparation of monodisperse CdS nanoparticles by the size-selective photoetching technique. The characterization of the CdS nanoparticles obtained by the monochromatic light having the shorter wavelength has revealed the validity of the size-selective photoetching as the method for the preparation of the ultrasmall semiconductor nanoparticles in the cluster molecule regime. Experimental Section Methyl viologen dichloride and cadmium perchlorate were purchased from Tokyo Chemical Industry and Mitsuwa Pure Chemicals, respectively. Other chemicals used in this study were purchased from Wako Pure Chemicals Industries. Water used for the preparation of all aqueous solutions was a water purified with use of a Millipore Milli-Q system. The size-selective photoetching of CdS nanoparticles was performed by using methods similar to those reported in our previous papers.57,59,60 The original CdS nanoparticles stabilized with sodium hexametaphosphate (HMP) were synthesized by an injection of a stoichiometric amount of H2S gas into 500 cm3 of aqueous solution containing 2.0 × 10-4 mol dm-3 Cd(ClO4)2 and 1.0 × 10-4 mol dm-3 HMP at pH 10.3, followed by stirring for 1 h under nitrogen atmosphere. The obtained CdS nanoparticles had the average size of 6.5 nm with a standard deviation of 1.5 nm, as previously reported in our papers.57 The light sources used in this study were an argon ion laser (Ion Laser Technology, Model 5500A) and a 500-W high-pressure mercury arc lamp (Ushio, model USH500D). The former was used for the irradiation of the monochromatic light at 488.0 and 457.9 nm with the light intensity of 20 and 3.2 mW cm-2, respectively. On the other hand, when the Hg lamp was used as a light source, the emission lines at 436, 405, and 365 nm with the irradiation intensities of 5.9, 4.2, and 1.8 mW cm-2, respectively, were obtained using a monochromator. The full width at half-maximum intensity of the Hg emission lines was about 5 nm, irrespective of the wavelength of monochromatic light. The irradiated area of the cell was fixed to about 3 cm2. A 30 cm3 portion of 2.0 × 10-4 mol dm-3 HMP-stabilized CdS colloid was put in a quartz cell and methyl viologen dichloride was added to the cell to give a concentration of 5.0 × 10-5 mol dm-3. After oxygen gas was bubbled for 5 min, the colloid was irradiated with monochromatic light of 488.0 nm under vigorous stirring. Absorption spectra of the CdS nanoparticles were measured intermittently during the course of irradiation using a photodiode array spectrophotometer (Shimadzu, MultiSpec-1500). When a steady-state absorption spectrum was obtained, the wavelength of the irradiated light was changed to the next shorter wavelength to obtain another blue-shifted steady spectrum due to the resulting smaller CdS particles. By repeating these procedures with use of the above-mentioned monochromatic light, we finally obtained the steady-state absorption spectra of CdS nanoparticles stabilized with HMP by the irradiation at 365 nm. In the case of further irradiation at 355 nm, a 2.0 cm3 portion of CdS colloids was put in the quartz cell (1 × 1 × 4 cm) and irradiated by using the third harmonic (355 nm) of a Q-switched Nd: YAG laser (Spectra Physics, GCR-11, pulse width 7 ns) at a frequency of 10 Hz with light intensity of 14 mW cm-2. The surfaces of the CdS nanoparticles were modified with thiophenol. A 30 cm3 portion of monodisperse CdS nanoparticle colloid obtained by the size-selective photoetching was added to 15 cm3 of ethanol containing 1.5 × 10-2 mol dm-3 thiophenol and 1.5 × 10-2 mol dm-3 tetramethylammonium chloride (Me4-
J. Phys. Chem. B, Vol. 105, No. 29, 2001 6839 NCl). The CdS colloid was bubbled with nitrogen gas for 30 min, followed by agitation for 48 h. By concentrating the solution to about one-fourth under vacuum, thiophenol-modified CdS nanoparticles were precipitated. The obtained particles were washed with water, methanol, and acetonitrile many times. Finally, the obtained CdS nanoparticles were purified by precipitation with addition of acetonitrile after they were dissolved in N,N-dimethylformamide (DMF). The obtained thiophenol-modified CdS nanoparticles were suspended in acetonitrile and stored under the nitrogen in the dark. The amount of samples needed for 1H NMR measurements was obtained by repeating the above-mentioned preparation procedures. 1H NMR spectra of thiophenol-modified CdS nanoparticles were obtained at 25 °C in N,N-dimethylformamide-d7 using a JEOL JNX-GSX 400 at 400 MHz with a Si(CH3)4 internal reference. To determine the chemical composition of the thiol-modified CdS nanoparticles, weight fractions of C, H, and N atoms in particles were determined by elemental analyses and that of Cd by atomic absorption spectrometry. The fraction of S atoms was determined by the ratio of the amount of S to that of Cd, which was obtained by energy-dispersive X-ray microanalyses. The size distribution of CdS nanoparticles was determined by a Hitachi H-9000 transmission electron microscope (TEM) operated at 300 kV. Specimens used for the TEM observation were prepared by dropping the thiophenol-modified CdS nanoparticles dissolved in DMF onto a Cu TEM grid with amorphous carbon overlayers, followed by drying under vacuum. The very dilute solutions of thiophenol-modified CdS nanoparticles were applied to the preparation of the specimens to prevent coalescence between the nanoparticles during TEM observations. To minimize ambiguity of boundaries between the nanoparticles and the amorphous carbon substrate, only TEM images of the CdS nanoparticles having a clear lattice fringe were selected in order to obtain the size distribution. The CdS cluster molecules of (Me4N)2[Cd(SC6H5)4] and (Me4N)2[Cd4(SC6H5)10] were chemically synthesized according to literature procedures.41,42,46 These compounds were identified by 1H NMR spectra, elemental analyses, and absorption spectra. Results and Discussion The Wavelength Region of the Monochromatic Light Available for the Size-Selective Photoetching. Figure 1 shows absorption spectra of CdS nanoparticles colloid with irradiation of monochromatic light. It was found that the absorption spectra were successively blue-shifted and their shapes became more structured with a decrease in the wavelengths of irradiation light. Furthermore, the absorption onset of each spectrum agreed well with the wavelength of irradiation light used, suggesting that the photocorrosion reaction of CdS nanoparticles proceeded until the photoexcitation of CdS nanoparticles could not occur due to an increase in energy gap with a decrease in the particle size. Such photocorrosion behavior was more clearly recognized if the transition of the absorption spectra was monitored during the course of the light irradiation, as shown in Figure 2 a, where changes in the absorption spectra caused by irradiation of the 365-nm light were shown. The CdS particles used here were previously submitted to the size-selective photoetching with the 405-nm light. The exciton peak was gradually blue-shifted and the steady-state spectrum was obtained when the absorption at 365 nm became almost zero. However, completely different behavior was met if irradiation of 355-nm light was made to the CdS nanoparticles that were prepared by the size-selective photoetching with 365-nm light, as shown in Figure 2b. The
6840 J. Phys. Chem. B, Vol. 105, No. 29, 2001
Figure 1. Absorption spectra changes of HMP-stabilized CdS colloid caused by irradiation with various wavelength: (a) original CdS, and CdS nanoparticles after irradiation at (b) 488.0, (c) 457.9, (d) 436, (e) 405, and (f) 365 nm.
Figure 2. Changes in the absorption spectra during the size-selective photoetching with irradiation at 365 (a) and 355 nm (b). The initial CdS nanoparticles were prepared by the size-selective photoetching with irradiation at 405 (a) and 365 nm (b). The irradiation time (min) is indicated in the figures.
absorption onset was almost unchanged, then the decrease in the absorbance of whole wavelength region occurred completely, indicating that all CdS nanoparticles were photocorroded to null, while the first exciton peak seemed slightly blue-shifted. This behavior was observed with independence of the irradiation light intensity from 14 to 110 mW cm-2. Furthermore, any monochromatic light having wavelength shorter than 355 nm also caused the complete photocorrosion of CdS nanoparticles. These results indicated that the CdS nanoparticles photoetched by 355-
Torimoto et al. nm irradiation were unstable, so they coalesced with each other to give bigger particles which were subjected to further photocorrosion. So we concluded that the size-selective photoetching with irradiation at 365 nm produced the smallest CdS nanoparticles. Since it has been reported that CdS cluster molecules and ultrasmall CdS nanoparticles had the preferential sizes to be stabilized, which decreased stepwise in association with the discontinuous blue-shift of absorption spectra,23,40,46 the instability of the photoetched CdS nanoparticles shown in Figure 2b suggested that the further photoetching of CdS nanoparticles with use of lights having the wavelengths shorter than 365 nm was not appropriate for inducing the discontinuous decrease in the particles size. The Thiophenol Modification of the CdS Nanoparticles Prepared by the Size-Selective Photoetching. Figure 3 shows the absorption spectra of CdS nanoparticles, surfaces of which were modified with thiophenol. Also included in this figure are those of HMP-stabilized CdS nanoparticles before the surface modification. It was found that both the first exciton peak and the absorption onset of the CdS nanoparticles prepared by the irradiation at 405 and 365 nm were largely red-shifted by thiophenol modification, while there were few changes in the absorption spectra with surface modification in the case of original CdS nanoparticles and those prepared by 457.9 nm. The wavelength of the first exciton peaks of CdS nanoparticles is summarized in Table 1. It was found that the degree of the red-shift of the exciton peaks with thiophenol modification was enhanced with a decrease in the CdS nanoparticle size. One might think that these red shifts of the absorption spectra resulted from the coalescence of CdS nanoparticles, which caused the decrease in the energy gap of CdS nanoparticles. TEM observation did not give clear images of HMP-stabilized CdS nanoparticles that were irradiated at wavelengths shorter than 405 nm, because of both the very low concentration of the resulting CdS nanoparticles and the coexistence of a great amount of HMP used as a stabilizer, so we could not compare directly the size distribution of thiophenol-modified CdS nanoparticles with those obtained without surface modification. If the coalescence of CdS nanoparticles occurred, it would be expected that the size distribution of CdS nanoparticles became large. However, since the very narrow distribution was observed for the thiophenol-modified CdS nanoparticles, as will be mentioned below, the incidence of the coalescence during the surface modification procedure must be small, if present at all. A possible explanation for the peak shift observed in Figure 3 is changes in the energy structure of CdS nanoparticles caused by the electronic interaction between the aromatic rings of the bound thiophenol molecules and the CdS core. This view was supported by the findings reported in our previous paper,59 where the surface modification with 2-aminoethanethiol did not cause any serious changes in the absorption spectra if CdS nanoparticles were prepared by size-selective photoetching with the monochromatic light having the wavelengths of 410, 430, and 450 nm. Nosaka and co-workers pointed out the similar absorption shifts of CdS nanoparticles caused by the surface modification with thiophenol.8,40 Also they showed that the exciton peaks of thiophenol-modified CdS nanoparticles were located at longer wavelength than those of the corresponding alkylthiol-modified nanoparticles. From those facts it was concluded that there was the strong electronic interaction between the modified thiophenol and CdS core, its magnitude being enhanced with a decrease in the particle size. Furthermore, Weller and co-workers have reported the changes in the wavelength of exciton peak of the CdS cluster molecules with
Characterization of Ultrasmall CdS Nanoparticles
J. Phys. Chem. B, Vol. 105, No. 29, 2001 6841
Figure 3. Absorption spectra of CdS nanoparticles before (dotted line) and after thiophenol modification (solid line). Original CdS particles (a), and CdS nanoparticles prepared by irradiation of monochromatic light at 457.9 (b), 405 (c), and 365 nm (d). Spectra of thiophenol-modified CdS nanoparticles were taken in DMF and those of HMP-stabilized CdS nanoparticles were taken in aqueous solution containing 1.0 × 10-4 mol dm-3 HMP.
TABLE 1: Exciton Peaks of CdS Nanoparticles and the Average Size (dav) and Standard Deviation (σ) of the Thiophenol-Modified CdS Nanoparticles exciton peak/nm λirrad/nma
HMP-stabilized
thiophenol-modified
dav/nm
σ/nm
none 457.9 405 365
460 439 379 339
460 441 393 359
6.5 2.8 2.2 1.7b
1.5 0.18 0.11 0.19
a The irradiation wavelength of monochromatic light. b The size of CdS nanoparticles at the maximum frequency in the size distribution shown in Figure 7.
dependence of the kinds of the capping agents,50 where Cd32S14(SCH2CH(OH)CH3)36‚4H2O had the first exciton peak at 325 nm, which was blue-shifted from that of Cd32S14(SC6H5)36‚4DMF at 358 nm,48 despite the exactly same CdS core. Figure 4 shows 1H NMR spectra of thiophenol-modified CdS nanoparticles in DMF. This figure also contained those of the CdS cluster molecules of [Cd(SC6H5)4]2- and [Cd4(SC6H5)10]2-. 1H NMR spectra in Figure 4 did not exhibit any change if the dissolution and precipitation of nanoparticles were repeated several times during purification procedures, and furthermore, there were no signals at 7.36 (d, 2H), 7.26 (dd, 2H), and 7.13 ppm (t, 1H) in the 1H NMR spectra that were attributed to the ortho-, meta-, and para-protons of free thiophenol molecules in DMF, respectively. These facts indicated that the observed signals were assigned to thiophenol chemically bound on the CdS surface that could not be liberated when the CdS nanoparticles were dissolved in DMF solution. The 1H NMR signals obtained for thiophenol bound on the original CdS nanoparticles were roughly classified into two groups. One was a set of large
signals at 7.59 (d), 7.43 (dd), and 7.34 (t) ppm, which were attributable to the ortho-, meta-, and para-protons of the bound thiophenol, respectively, and the other was a set of small multiplet signals around 7.78, 7.72, and 7.30 ppm, which were difficult to assign. Almost similar spectra were observed for the case of CdS nanoparticles prepared by irradiation at 457.9 nm, indicating that a large fraction of the bound thiophenol on CdS nanoparticles had the uniform chemical environment, which was not seriously varied if the size of the CdS nanoparticles decreased to that obtained by 457.9-nm irradiation. In contrast, the signals observed for original CdS nanoparticles disappeared in the 1H NMR spectra of CdS nanoparticles prepared by the irradiation at 365 nm, and the new three broad signals emerged at 7.42 (d, 2H), 6.97(dd, 2H), and 6.88 (t, 1H) ppm. The CdS nanoparticles prepared by the 405-nm irradiation exhibited the intermediate 1H NMR spectrum, which had the signals observed for both the original CdS nanoparticles and the CdS nanoparticles prepared by 365-nm irradiation. On the other hand, 1H NMR spectra of Cd4(SC6H5)102- cluster molecules showed three sharp signals at 7.45 (d, 2H), 6.92 (dd, 2H), and 6.79 (t, 1H) ppm and those of Cd(SC6H5)42- cluster molecules appeared at 7.47 (d, 2H), 6.89(dd, 2H), and 6.75 (t, 1H) ppm. By comparing these facts, it was found that the smallest CdS nanoparticles had the bound thiophenol molecules whose chemical environment was similar to that of CdS cluster molecules. Figure 5 shows the emission and excitation spectra at 77 K of the thiophenol-modified CdS nanoparticles powders prepared by 365-nm irradiation. The broad emission band appeared with its peak around 500 nm, while no emissions were observed at room temperature in DMF solution. If the position of the emission peak was compared with that of the absorption threshold, it was found that the emission resulted from the defect
6842 J. Phys. Chem. B, Vol. 105, No. 29, 2001
Torimoto et al.
Figure 5. Emission (solid line) and excitation (dotted line) spectra at 77 K of the smallest thiophenol-modified CdS nanoparticles powders prepared by 365-nm irradiation. The excitation wavelength for the emission spectra was 350 nm, and the monitoring wavelength for the excitation was 500 nm.
Figure 4. 1H NMR spectra of thiophenol-modified CdS nanoparticles and CdS cluster molecules in DMF. Original CdS (a), CdS nanoparticles prepared by irradiation of monochromatic light at 457.9 (b), 405 (c), and 365 nm (d), and CdS cluster molecules of [Cd4(SC6H5)10]2- (e) and [Cd(SC6H5)4]2- (f).
sites or the lower lying excited states but not from band-toband transitions. The excitation spectrum shows two peaks at 353 and 290 nm, which are assumed to correspond to transitions of the first exciton state and the higher states, respectively. The position of the former peak was in good agreement with that observed in the absorption spectra in DMF (Figure 3d), while the latter could not be observed in the solution due to disturbance of the large absorption of DMF. It has been reported that Cd32S14(SC6H5)36‚4DMF dissolved in tetrahydrofuran exhibited the broad emission with its peak around 500 nm at 77 K and their excitation spectra had two peaks at 366 and 313 nm, while the lowest absorption band was located at 358 nm.48 By comparing the spectra data of this cluster with the results in Figure 3d and 5, it was noted that the energy structure of the smallest thiophenol-modified CdS nanoparticles seemed to be very similar to that of Cd32S14(SC6H5)36‚4DMF, which was one of the largest CdS cluster molecules reported to date. Characterization of the Thiophenol-Modified CdS Nanoparticles Prepared by 365-nm Irradiation. Figure 6 shows the typical high-resolution TEM images of individual thiophe-
Figure 6. TEM image of a typical CdS nanoparticle modified with thiophenol. CdS nanoparticles prepared by irradiation of monochromatic light at 457.9 (a), 405 (b), and 365 nm (c, d). The bars indicated in the pictures represent a length of 2 nm. Lattice fringes observed in a-c and d were assigned to (111) and (220) planes, respectively.
nol-modified CdS nanoparticles. Regardless of the wavelengths used for the size-selective photoetching, we observed CdS nanoparticles displaying the clear lattice fringes with the interplanar spacing of 0.33 or 0.21 nm assigned to the (111) or (220) planes of cubic CdS crystal structure, respectively. The shape of the larger CdS nanoparticles prepared by the irradiation with wavelengths longer than 405 nm was almost spherical, as
Characterization of Ultrasmall CdS Nanoparticles
J. Phys. Chem. B, Vol. 105, No. 29, 2001 6843 TABLE 2: Molar Ratios of S2-/Cd2+ and C6H5S-/Cd2+ and the Number of CdS(111)-like Lattice Planes in a Cluster Molecule
C6H5S-/Cd2+ S2-/Cd2+ thiophenol-modified CdS nanoparticlesc [Cd4(SC6H5)10]2[Cd10S4(SC6H5)16]4[Cd17S4(SC6H5)28]2Cd32S14(SC6H5)36·4DMF
charge balancea
number of lattice planesb
1.22
0.410
1.02
5d
2.50 1.60 1.65 1.13
0 0.400 0.235 0.438
1.25 1.20 1.06 1.00
2 3 4 5
a Calculated by (2S2- + C H S-)/2Cd2+. b The number of the lattice 6 5 planes in a cluster molecule, whose interplanar spacing corresponds to that of the (111) plane in a cubic CdS crystal structure. c CdS nanoparticles were prepared by 365 nm-irradiation. d The number of (111) planes most frequently observed in the smallest thiophenolmodified CdS nanoparticles, as shown in Figure 7.
Figure 7. Distribution of the number of (111) planes observed in TEM images of thiophenol-modified CdS nanoparticles prepared by 365nm irradiation.
shown in Figure 6a,b. In the case of CdS nanoparticles prepared by 365-nm irradiation, however, markedly different shape of nanoparticles was observed, as shown in Figure 6c,d, where CdS nanoparticles exhibited an edged pyramidal shape. Considering that the several kinds of the structurally well-characterized CdS cluster molecules, such as [Cd4(SC6H5)10]2-,41,42 [Cd10S4(SC6H5)16]4-,42 [Cd17S4(SC6H5)28]2-,43 and Cd32S14(SC6H5)36‚4DMF,48 had tetrahedral or truncated tetrahedral shapes, these TEM images might reflect the tetrahedral geometry of nanoparticles, suggesting that the smallest CdS nanoparticles belonged to the same families as tetrahedral CdS cluster molecules. The selected-area electron diffraction patterns of the CdS nanoparticles simultaneously obtained in TEM observations showed clearly only four diffraction rings, corresponding to the interplanar spacing of 3.39, 2.07, 1.75, and 1.19 Å, which were assignable to diffractions from the (111), (220), (311), and (422) planes of a cubic crystal structure of CdS, respectively,62 regardless of the size of the thiophenol-modified CdS nanoparticles. These facts indicated that the size-selective photoetching technique proved not to influence the crystal structure of the resulting nanoparticles at all, even though the size of the photoetched CdS nanoparticles became extremely small. Since it was very difficult to observe the surface structures of CdS nanoparticles with the electron diffraction measurements, however, it could not be ruled out that a part of the surface structures formed by the thiophenol modification might have hexagonal phase, as reported in the case of the Cd32S14(SC6H5)36‚4DMF cluster,48 which has the core of the cubic CdS structure and the four tetrahedral corners of the (µ-SR)3Cd structure similar to the hexagonal CdS phase. Figure 7 shows the distribution of the number of (111) planes observed in TEM images of a thiophenol-modified CdS nanoparticles prepared by 365-nm irradiation. In this figure, we also estimated the size of nanoparticles by multiplying the number of (111) planes by the interplanar spacing of them, 0.336 nm.62 As recognized, five or six lattice planes assigned to (111) planes were observed in most CdS nanoparticles and nearly 70% of the nanoparticles had five lattice planes, which corresponded to a particle size of about 1.7 nm. Table 1 shows the size distribution profiles of the thiophenol-modified CdS nanoparticles determined by TEM observations. It was found that the average diameter decreased with a decrease in the wavelength
of irradiation light, agreeing with the results of absorption spectra. Furthermore, the size distribution of the photoetched CdS nanoparticles was not seriously changed by the wavelength of irradiation light used for the photoetching. These facts indicated that the size-selective photoetching technique allowed us to control the desired size of nanoparticles without losing their monodispersibility until the size of CdS nanoparticles decreased to 1.7 nm. The elemental analyses of the smallest thiophenol-modified CdS nanoparticles that were prepared by 365-nm irradiation gave the results C, 35.10; H, 2.96; N, 0.95; Cd, 40.61; and S, 18.9 wt % (total 98.5 wt %). Since 1H NMR spectra of thiophenolmodified CdS nanoparticles in DMF showed the signal at 3.39 ppm assigned to methyl protons in Me4N+ ions (not shown), it was reasonable to assume that all of the N atoms resulted from Me4N+ ions contained in the resulting CdS nanoparticles. If the weight fractions of C and H derived from Me4N+ were subtracted from the results of the elemental analyses, the ratio of the resulting weight fractions of C to those of H were in good agreement with that expected from the chemical composition of thiophenolate ion within the experimental errors. Table 2 gives the molar ratios of S2-/Cd2+ and C6H5S-/Cd2+ obtained for the thiophenol-modified CdS nanoparticles prepared by 365nm irradiation and those calculated for structurally wellcharacterized thiophenol-capped CdS molecular clusters.41-43,48 These values have been reported to be greatly influenced by the size of the thiophenol-modified CdS nanoparticles,23 where the molar ratio of S2-/Cd2+ increased and that of C6H5S-/Cd2+ decreased with an increase in the size of CdS cluster molecules, being in good agreement with the changes in the surface-tovolume ratio. If the comparisons were made between the smallest CdS nanoparticles and the CdS cluster molecules, it was found that both molar ratios of S2-/Cd2+ and C6H5S-/Cd2+ of the smallest CdS nanoparticles agreed well with those of Cd32S14(SC6H5)36‚4DMF, except for a little discrepancy in the charge balance of the particle. Also included in Table 2 is the number of the lattice planes in a cluster molecule, whose interplanar spacing corresponds to that of the (111) plane in a cubic CdS crystal structure. It is noteworthy that only Cd32S14(SC6H5)36‚4DMF cluster molecule contained five lattice planes, and this value was in agreement with the number of (111) planes most frequently observed in the smallest CdS nanoparticles, as shown in Figure 7. Considering the similarity in the chemical compositions and the particle size as well as resemblance of the spectra data shown in Figure 5, we concluded that the smallest thiophenol-modified CdS nanoparticles prepared by
6844 J. Phys. Chem. B, Vol. 105, No. 29, 2001 365-nm irradiation had a molecular structure similar to that of Cd32S14(SC6H5)36‚4DMF. Conclusion The present study showed one successful approach to the preparation of the ultrasmall CdS nanoparticles in the cluster molecule regime by the size-selective photoetching technique. With a decrease in the wavelength of irradiation light, the size of the CdS nanoparticles decreases and finally reaches the smallest size of about 1.7 nm with the monochromatic light irradiation at 365 nm. The smallest CdS nanoparticles modified with thiophenol have a molecular structure similar to that of the largest CdS cluster molecules, Cd32S14(SC6H5)36‚4DMF. Since the size-selective photoetching can be principally applied to the preparation of any semiconductor nanoparticles that are photocorroded under irradiation, the results of the present study will open the way to preparation of various kinds of ultrasmall semiconductor nanoparticles having sizes in the cluster molecule regime that have never been prepared by chemical syntheses methods. Another interesting feature obtained in this study is that the absorption shift of the CdS nanoparticles by the thiophenol modification remarkably appeared due to the electronic interaction between the CdS core and the bound thiophenol molecules when the particle size was less than 2.8 nm. This indicates that there is the possibility to modulate the energy structure of the ultrasmall nanoparticles by changing their surface modification conditions. Accordingly, it will be a significant future subject to investigate the influence of changes in both the kinds of surface modifiers and the surface structures on the physicochemical properties of ultrasmall semiconductor nanoparticles. Acknowledgment. This research was partially supported by the Mazda Foundation’s Research Grant and by Grant in Aid for Encouragement of Young Scientists (No. 12750733) from Japan Society for the Promotion of Science. References and Notes (1) Henglein, A. Chem. ReV. 1989, 89, 1861-1873. (2) Wang, Y.; Herron, N. J. Phys. Chem. 1991, 95, 525-532. (3) Kamat, P. V. Chem. ReV. 1993, 93, 267-300. (4) Weller, H. Angew. Chem., Int. Ed. Engl. 1993, 32, 41-53. (5) Hagfeldt, A.; Gra¨tzel, M. Chem. ReV. 1995, 95, 49-68. (6) Nozik, A. J.; Memming, R. J. Phys. Chem. 1996, 100, 1306113078. (7) Alivisatos, A. P. Science 1996, 271. (8) Nosaka, Y. Curr. Top. Colloid Interface Sci. 1997, 1, 225-233. (9) Nirmal, M.; Brus, L. Acc. Chem. Res. 1999, 32, 407-414. (10) Empedocles, S. A.; Neuhauser, R.; Shimizu, K.; Bawendi, M. G. AdV. Mater. 1999, 11, 1243-1256. (11) Eychmu¨ller, A. J. Phys. Chem. B 2000, 104, 6514-6528. (12) Klimov, V. I. J. Phys. Chem. B 2000, 104, 6112-6123. (13) Steigerwald, M. L.; Alivisatos, A. P.; Gibson, J. M.; Harris, T. D.; Kortan, R.; Muller, A. J.; Thayer, A. M.; Duncan, T. M.; Douglass, D. C.; Brus, L. E. J. Am. Chem. Soc. 1988, 110, 3046-3050. (14) Kortan, A. R.; Hull, R.; Opila, R. L.; Bawendi, M. G.; Steigerwald, M. L.; Carroll, P. J.; Brus, L. E. J. Am. Chem. Soc. 1990, 112, 13271332. (15) Herron, N.; Wang, Y.; Eckert, H. J. Am. Chem. Soc. 1990, 112, 1322-1326. (16) Nosaka, Y.; Yamaguchi, K.; Miyama, H.; Hayashi, H. Chem. Lett. 1988, 605-608. (17) Nosaka, Y.; Ohta, N.; Fukuyama, T.; Fujii, N. J. Colloid Interface Sci. 1993, 155, 23-29. (18) Nosaka, Y.; Ohta, N.; Miyama, H. J. Phys. Chem. 1990, 94, 37523755. (19) Rajh, T.; Micic, O. I.; Nozik, A. J. J. Phys. Chem. 1993, 97, 1199912003. (20) Micic, O. I.; Curtis, C. J.; Jones, K. M.; Sprague, J. R.; Nozik, A. J. J. Phys. Chem. 1994, 98, 4966-4969.
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